This invention relates generally to a differential pressure transmitter of the force-beam type which operates on the open loop principle and includes a displacement transducer having an S-shaped magnetostriction wire, and more particularly to an arrangement which ensures a linear response of the transducer.
A differential pressure transmitter is used to measure various process variables such as flow rate, liquid level and density when these process variables are convertible to a differential pressure signal. One important application of the force-balance principle is in a differential-pressure transmitter. Such transmitters are widely used in industrial process systems to produce an output signal suitable for transmission to a remote station for operating indicator and automatic process control equipment.
In a transmitter of this type, an elongated force bar is pivoted about a transverse axis and an input force derived from a differential pressure capsule and corresponding to the flow rate of the fluid to be measured is applied to the force bear to produce a torque about its fulcrum. Also applied is a rebalance torque which tends to hold the bar motionless.
The rebalance torque is developed by a negative feedback loop that includes a detector to sense any slight change in force bar position due to an unbalance of torques. The detector generates a corresponding feedback signal that is directed to a feedback motor. The motor, in turn, applies to the force bar a force in opposition to the input force. This feedback signal is maintained proportional to the differential pressure being measured and is usable to produce an output signal for transmission to a remote control station or to an indicating or recording device.
In a force-balance instrument of the electrical type, such as that disclosed in U.S. Pat. No. 3,832,618, the feedback system is provided with an electric motor and the output signal is electrical in nature, whereas in the pneumatic type, such as that disclosed in U.S. Pat. No. 3,742,969, the motor is in the form of a pneumatically-actuated bellows and the output signal which is applied to the bellows is fluidic in nature.
In a force-balance transmitter, there is virtually no movement of the force bar over the full-scale range of operation. This virtual absence of movement is highly advantageous, for it effectively eliminates non-linearity and other errors of the type encountered in so-called motion-balance instruments.
A differential-pressure force-balance transmitter of the type disclosed in the above-cited patents acts on a closed-loop principle in that the input force applied to the force bar is balanced by a feedback force applied thereto through a feedback loop.
Also known are electronic differential pressure transmitters of the so-called open-loop type. In one form of an open-loop differential transmitter, the low and high pressure fluid inputs are applied to a pair of process diaphragms which are hydraulically coupled to opposite sides of an electrode diaphragm. The deflection of the electrode diaphragm depends, therefore, on the difference between the low and high input pressures. This deflection is converted into a corresponding change in the capacitance established between the electrode diaphragm and the meter body. The change in capacitance is detected by a capacitance bridge and amplified to generate an output signal which represents differential pressure.
The advantage of an open-loop differential pressure transmitter of the above-described capacitive transducer type over a standard, closed-loop transmitter of the force-balance type is that the former is not only relatively light-weight and less expensive to manufacture, but it is also substantially insensitive to mechanical vibration. On the other hand, the closed loop force-balance type is characterized by high accuracy, a wide zero suppression range and the absence of electrical connections in the meter body.
In the above-identified copending patent application of Kazahaya, Ser. No. 579,712, filed May 21, 1975, now U.S. Pat. No. 3,968,693, the disclosure of which is incorporated herein by reference, there is shown an open-loop differential pressure transmitter which possesses the advantages both of a closed-loop and of an open-loop transmitter without certain drawbacks characteristic of known devices of these types. Moreover, since the Kazahaya transmitter does not include a pneumatic or electric-feedback motor, meter bodies of the type currently used either in a pneumatic or in an electric differential-pressure transmitter may be used for the open-loop force-beam instrument.
In the Kazahaya transmitter, a force beam is pivotally mounted on the housing of the meter body, the lower end of the beam below the fulcrum lying within the body chamber and the upper end of the beam above the fulcrum extending outside of the body. An input force is applied to the lower end of the beam and deflects the beam to an extent that depends on the flow rate or other parameter being measured.
Linked to the upper end of the beam and external to the meter body is a displacement transducer adapted to convert the deflection of the beam to a corresponding electrical signal suitable for transmission. In a preferred embodiment of the Kazahaya transducer, use is made of an S-shaped magnetostriction wire whose radius of curvature is varied as a function of beam displacement. This S-shaped wire functions as an electro-mechanical resonator which is caused to oscillate torsionally at its fundamental frequency. When the transducer is subjected by the beam to a displacement in the axial direction, the frequency of oscillation changes as a function of displacement.
The frequency of oscillation of the transducer is therefore an index of displacement and represents the output signal. It has been found that the relationship between the frequency of the output signal and displacement exhibits non-linear deviations. The non-linearity in this relationship can be larger than 1.0% of the transducer span. This margin of error may be acceptable in some differential-pressure transmitter applications, but it is not tolerable in applications involving automatic industrial process control systems wherein the transmitter acts to sense a process variable to produce a signal for transmission to a process control mechanism.